In large circuits, it is often desirable to use a single, central, reference voltage source, in order to save area and/or save power. A single central reference voltage source is also known as a global reference voltage. In the context of this document, the term area generally refers to the area used by circuit elements. In the case of an integrated circuit (IC), area is known as real estate, and generally refers to the surface area of an integrated circuit die. A challenge in using a central reference voltage source is accurately distributing the reference voltage across relatively large distances. In the context of this document, accurate distribution refers to the value of a reference voltage at the output of a central reference voltage source being substantially equal to the value of the voltage at an input to a local reference voltage generator. Distribution can introduce voltage errors due to ground and supply voltage differences across the circuit, and stray voltages that are coupled inductively and/or capacitively to the conductors used to distribute the reference voltage.
In the context of this document, unless otherwise specified, the term “refer”, when applied to voltages, means the difference between a first voltage and a second voltage. For example, if a first voltage is 2.0V referred to a second voltage, and the second voltage is 0.1 V above earth ground, the first voltage is 2.1 V above earth ground.
In the context of this document, unless otherwise specified, resistance values are given in Ohms. “k” indicates multiplication by 1000. For example, “2 k” indicates a resistance of 2000 Ohms.
In the context of this document, as is popular in the industry, conductors are also referred to as lines and the terms should be understood to be used interchangeably unless specified otherwise.
A variety of conventional techniques attempt to provide differential voltage references. U.S. Pat. No. 5,821,807 to Brooks teaches a differential voltage reference circuit implemented in CMOS that provides a continuous differential voltage having good substrate and power supply noise rejection and low power consumption. However, Brooks does not explain how the differential voltage can be used to provide a local voltage referenced to local ground or a local power supply voltage.
U.S. Pat. No. 4,926,138 to Castello et al introduces a fully differential voltage source. The voltage reference is obtained from a bandgap voltage source fed with currents proportional to the temperature, in order to minimize thermal voltage variations. However, Castello et al also does not offer a solution for distribution of the voltage source nor providing a local voltage referenced to local ground or a local power supply voltage.
U.S. Pat. No. 5,939,867 to Capici et al teaches a circuit that can generate a local voltage with respect to a single input reference voltage. The circuit of Capici et al is used for local connection and operation.
U.S. Pat. No. 6,885,236 to Vorenkamp teaches a circuit capable of generating an output voltage with respect to two voltages that differ by a reference voltage. The circuit of Vorenkamp is described as having a reference generator that generates a voltage reference. This generic voltage source directly supplies a voltage reference to a single circuit, which Vorenkamp describes in detail.
While the above-described conventional circuits can be used as components of a system for distributing a central master reference voltage to local circuits, there remains the problem of how to distribute accurately a master reference voltage to local circuits.
U.S. Pat. No. 5,506,541 to Herndon, titled Bias Voltage Distribution System, teaches a system for generating bias potentials at one main location within a logic circuit and then distributing them to other logic circuits. As is common in the art, this system is an example of sending voltages from a central source to remote circuits. While many systems generate signals at a central source and receive signals at a remote circuit, there remains the problem of how to distribute accurately a master reference voltage to local circuits
There is thus a widely recognized need for, and would be highly advantageous to have, a system and method for distributing accurately a single central master reference voltage to a plurality of local circuits. It is further desirable to be able to generate a local reference voltage based on the master reference voltage and a local voltage source (for example local ground or a local power supply voltage).